Fix Documentation for async iterators in README

[janpaepke]

This is the current example for async iterators:

// Iterate over all payments.
for await (let payment in mollieClient.payments.iterate()) {
  // (Use break to end the loop prematurely.)
}

This has two issues, one being a mistake and one being a style issue.

1. Mistake: It should be of rather than in.
2. Style: The example should recommend using const rather than let.

Usually when iterating over payments we do not expect them to be reassigned within the for block.
If anyone would for some reason, it would be specific to their implementation and they would know to change it to 'let'.
Preferring const here is in line with the prefer-const rule from eslint.

[edorivai]

LGTM